Heat transfer control for a prosthetic retinal device

ABSTRACT

A method for controlling heat dissipated from a prosthetic retinal device is described. A heat transfer device employs the Peltier heat transfer effect to cool the surface of the retinal device that faces the retina by dissipating/transferring collected heat away from the retina and towards the iris or front of the eye. According to one embodiment, a heat pump is formed in a second substrate on the retinal device. The heat pump is controlled by a temperature sense device that activates the heat pump, when a first predetermined temperature limit is exceeded. The temperature sense device deactivates the heat pump, when a temperature of the retinal device drops below a second predetermined temperature. According to another embodiment, a supply current of the retinal device may pass through the heat pump and a direction of heat transfer by the heat pump can be reversed, when the first predetermined temperature is exceeded.

RELATED APPLICATION

This utility patent application is a divisional of U.S. patentapplication Ser. No. 10/995,047, now U.S. Pat. No. 7,306,621, filed Nov.19, 2004 and allowed Dec. 11, 2007, the benefit of which is herebyclaimed under 35 U.S.C. §120(e) and incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to eye prosthetics, and, in particular, toa method for controlling heat transfer from a prosthetic retinal deviceto eye tissue.

BACKGROUND

Various diseases, physical trauma, and birth defects can result in thedestruction or impaired functionality of rod and cone cells in theretina, which are the primary mechanism for converting incident lightinto electro-chemical signals that can be interpreted as sight by thebrain. Generally, loss of this functionality can not be mitigated byconventional surgical or pharmacological methods.

A prosthetic retinal device could be used to restore visual perceptionto a person suffering from damage to the retina due to birth defects,physical trauma, and/or disease such as retinitis pigmentosa, maculardegeneration, and the like. Some birth defects, trauma, or disease cancause destruction of the rods and cones in the retina, but leave otherretina cells such as ganglion cells largely intact. Consequently, theapplication of an electrical signal to these other cells in the retinacan still enable the perception of light even if the rod and/or conecells are impaired or absent. In the retina, ganglion cells translateelectrical stimulation into electrochemical messages which aresubsequently communicated to the visual cortex of the brain through theoptic nerve.

However, at least because of operational constraints such asintra-ocular temperature and/or pressure, physical size, physicalcontact with the retina, and power supply limitations, the resolution ofelectrical signals provided by a prosthetic retinal device may belimited. Thus, it is with respect to these considerations and othersthat the present invention has been made.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following drawings. In the drawings,like reference numerals refer to like parts throughout the variousfigures unless otherwise specified.

For a better understanding of the present invention, reference will bemade to the following Detailed Description of the Invention, which is tobe read in association with the accompanying drawings, wherein:

FIG. 1 illustrates a plan view of a human head;

FIG. 2 shows a cut-away view of an eye with an epiretinal prostheticdevice;

FIGS. 3A and 3B illustrate a block diagram of an epiretinal prostheticdevice;

FIGS. 4A and 4B show block diagrams of two embodiments of a temperaturecontrolled epiretinal prosthetic device positioned over the anteriorsurface of the retina;

FIG. 5 illustrates one embodiment of a heat pump;

FIG. 6 shows a flow diagram generally showing a process for controllinga temperature of a retinal prosthetic device; and

FIG. 7 illustrates a flow diagram generally showing a process forcontrolling a temperature control of a retinal prosthetic device, inaccordance with the present invention.

DETAILED DESCRIPTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, which form a part hereof, andwhich show, by way of illustration, specific exemplary embodiments bywhich the invention may be practiced. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art.Among other things, the present invention may be embodied as methods ordevices. Accordingly, the present invention may take the form of anentirely hardware embodiment or an embodiment combining software andhardware aspects. The following detailed description is, therefore, notto be taken in a limiting sense.

Briefly stated, the present invention relates to a method and apparatusfor a heat transfer device adapted for implementation with a prostheticretinal device. In different embodiments, this prosthetic retinal devicecan be positioned on the surface of the retina (epiretinal), within thelayers of the retina tissue (subretinal), or suspended within the eye atsome distance away from the retina.

The prosthetic retinal device can imitate at least some of theoperations of rod and cone cells such as providing electricalstimulation of ganglion cells in response to incident light. Generally,the prosthetic retinal device receives incident light information from aseparate (or integrated) camera or sensor and translates thisinformation into at least one electrical signal that is provided to andrecognizable by ganglion cells in the retina. Also, by generatingmultiple electrical signals, a retinal device can provide a plurality ofstimulation points (pixels of resolution) that can be interpreted by thebrain as a form of sight.

The electrical signals provided by a prosthetic retinal device can beconverted into electrochemical signals by ganglion cells and otherunderlying tissue structures for communication to the brain's visualcortex. These electrochemical signals are carried via the optic nerve tothe brain for interpretation as sight. However, since the intra-oculartemperature is generally less than the normal body temperature of 98.6degrees Fahrenheit, it is possible that a relatively small build-up ofheat in a prosthetic retinal device could jeopardize its chronicimplantation. In particular, an elevated intraocular temperature causedby heat build up in a retinal device can erode the normal functionalityof cells/tissue in the eye, increase the risk of an infection, andinduce the immune system to respond with one or more defense mechanismssuch as a fever, and the like. The inventive heat transfer devicepreserves intra-ocular temperature by enabling a prosthetic retinaldevice to dissipate heat away from retinal tissue/cells.

The inventive arrangement of components for dissipating/transferringheat generated at least in part by a prosthetic retinal device enablesmore power to be employed by the device without substantiallydeleterious results. For example, a prosthetic retinal device thatemploys more power to provide a higher resolution (multiple electricalsignals for multiple pixels) without substantially increasing the heatdissipated to the retina at the point of implantation can do so withoutcausing a substantial increase various risk factors.

One aspect of the present invention utilizes a heat transfer devicebased on the Peltier heat transfer effect to cool the retina or “back”side of the prosthetic retinal device by dissipating heat towards thepupil or “front end” of the eye. Typically, heat dissipated towards thepupil end of the eye may be more easily transferred outside of the bodythan heat that is dissipated towards the retina or “back” side of theeye. Generally, the temperature gradient between the vitreous humor(fluid inside the eyeball) and the outside environment is larger than atemperature difference between the vitreous humor and the retina tissue.

In one embodiment, a Peltier based heat transfer device is formed in asecond substrate on the surface of the prosthetic retinal device thatfaces away from the retina and towards the pupil. One aspect of theinvention provides for a temperature sense circuit that can activate theheat transfer device, if a predetermined temperature is exceeded.

According to another embodiment, a supply current for a prostheticretinal device may also pass through the heat transfer device andtransfer of heat is subsequently controlled by reversing this supplycurrent, if a predetermined temperature is exceeded.

While some embodiments may be implemented in a prosthetic retinaldevice, the invention is not so limited. For example, the inventive heatexchanger may be employed with a cortical implant device, a spinalimplant device, and the like. Furthermore, the invention may beimplemented in any electrical, mechanical, and electromechanical implantdevice, such as an artificial heart pump, a pacemaker, an insulin pump,and the like. It is understood that the invention may be implemented invirtually any implantable medical device where it is desirable tocontrol heat dissipation of the device in situ.

Operating Environment

FIG. 1 illustrates human head 100, where the present invention may beimplemented. Human head 100 includes, in addition to usual features suchas ears, mouth, nose, and the like, eyeball 104 and optic nerve 102.

Eyeball 104 resides in an ocular cavity and is connected to the brainthrough optic nerve 102. Optic nerve 102 receives electrochemicalsignals representing visual information from ganglion cells in eyeball104 to special regions of the thalamus and visual cortex in the brain.Optic nerve 102 also carries electrochemical signals from the brain tothe eye.

FIG. 2 illustrates eye 200 with epiretinal device 218. As shown, eye 200includes at least optic nerve 202, sclera 204, retina 206, vitreoushumor 208, lens 210, iris 212, pupil 214, cornea 216, and epiretinaldevice 218 disposed on the retina.

Optic nerve 202 is a bundle of about a million nerves that carryelectrochemical signals corresponding to visual information fromganglion cells in retina 206 to the brain. Sclera 204 a white,non-transparent tissue, surrounds cornea 216 and provides protection todelicate inner structures of eye 200.

Retina 206 is disposed on an inner back wall of eye 200. Retina 206includes several layers of specialized cells such as rods and cones,ganglion cells, optic nerve fibers, and the like. Rods and cones arephotoreceptor cells in a bacillary layer that receives incident lightthrough iris 212 and lens 210, and generate electrochemical signals inresponse to the received light. Typically, a human eye includesapproximately six to seven million cones which can sense at least one ofa red, blue, and green color. In contrast, the rods are far morenumerous in a human eye than cones, about 120 million. While not colorsensitive, the rods enable sight under dark, or scotoptic conditions. Asmentioned above, diseases such as retinitis pigmentosa, maculardegeneration, and the like may lead to damage of the retina layer thatincludes the rods and cones. In a healthy eye, the electrochemicalsignals generated by the rods and cones is captured by the ganglioncells in a different layer and transmitted to optic nerve fibers. Opticnerve fibers, distributed throughout retina 206, concentrate in oneregion and form optic nerve 202 connecting eye 200 to the brain.

Vitreous humor 208 is a gelatinous, clear liquid that fills the innerspace of eye 200 surrounded by retina 206 and lens 210. Vitreous humor208 enables the preservation of a round shape for eye 200, and helpsmaintain an inner temperature of eye 200 slightly below a bodytemperature. Vitreous humor 208 is also critical in maintainingintra-ocular pressure.

Lens 210 is an internal focusing element of eye 200. Lens 210 controlsabout one third of a refraction of light that enters eye 200. Lens 210is curved on both sides and attached to ciliary muscle at its top andbottom. A contraction and expansion of the ciliary muscle in response toa signal from the brain enables lens 210 to alter its shape and therebya focus of eye 200. Lens 210 comprises soft material that allows thealteration of its shape, also called accommodation. In addition tocontrolling the focus of eye 200, lens 210 also controls the refractionof incoming light by absorbing particular wavelengths more than others.

Iris 212 is located on the outside of lens 210 and is made of very finemuscular tissue. Iris 212, which gives the eye its color, has asubstantially round hole in its center. The hole is pupil 214. Pupil 214controls an amount of light that enters eye 200 through lens 210. A sizeof pupil 214 is managed by contraction and expansion of the musculartissue of iris 212. The size of pupil 214 changes based, in part, on anambient light level. A response of pupil 214 is partially based on astimulation of rods and cones of retina 206.

Cornea 216 is a clear tissue covering a front part of eye 200 includingiris 212. Cornea 216 is a main source of refraction (about two third).Cornea 216 does not include any blood vessels, and is made of five clearlayers of epithelium. Cornea 216's main task is to protect the eyeagainst injuries and to provide a barrier against infection.

Epiretinal device 218 can be arranged to provide electrical, mechanical,electromechanical, and the like, stimulation to ganglion cells of retina206. Epiretinal device 218 may also include electrical circuitry that isarranged to receive light information from a camera, photoelectricsensor, and the like, determine characteristics of electrical signals tobe generated in response to the received information and providestimulation to the ganglion cells in the form of electrical signals,mechanical stimulation, or some combination of both electrical signalsand mechanical stimulation.

Epiretinal device 218 may be manufactured employing a flexiblesemiconductor material which enables the device to have a curvedstructure that is suited for implantation in the eye. Although notshown, in addition to placement on the anterior surface of the retina,epiretinal device 218 may be positioned in the layers of the retina, orsuspended in vitreous humor 208 near the anterior surface of the retina,and the like.

System and Apparatus

FIGS. 3A and 3B illustrate block diagrams of two embodiments ofprosthetic retinal devices 318A and 318B. As shown, retinal devices 318Aand 318B can include temperature sense circuit 322, process and controlcircuitry 324, heat transfer device 326, and microprobes 328. Processand control circuitry 324 includes in one embodiment current supply 325Aand in another embodiment current supply 325B. Prosthetic retinal device318B further includes power source 327.

Prosthetic retinal devices 318A and 318B may include circuitry forreceiving and processing incident light information from an externalsource such as a camera, photoelectric sensor, and the like, andproviding electrical signals to microprobes 328, which deliver thesignals as electrical stimulation to the anterior surface of the retina.Prosthetic retinal devices 318A and 318B may further includemultiplexing circuitry, mechanical activation circuitry, and the like,which may be incorporated in process and control circuitry 324. In oneembodiment as shown in FIG. 3A, power may be provided to retinal device318A through photosensitive sensors disposed on a top surface of thedevice from a laser beam along with the optical signals, a battery, acombination of multiple energy sources, and the like. Current supply325A may provide a current as described below based on the energysource.

In another embodiment as shown in FIG. 3B, power may be provided toretinal device 318B through RF induction. Power source 327 may be anexternal power supply device that is arranged to provide power throughRF induction to current supply 325B, which provide the current toprocess and control circuitry 324 based on the inductively providedpower.

In one embodiment, prosthetic retinal devices 318A and 318B may bemanufactured from a flexible semiconductor that is suited forpositioning on the anterior surface of the retina. In anotherembodiment, retinal devices 318A and 318B may be suspended in thevitreous humor of the eye to avoid tearing and other damage to theretina. Additionally, to reduce a risk of infection, clotting, and thelike, retinal devices 318A and 318B may be coated with heparin, teflon,and the like.

Prosthetic retinal devices 318A and 318B may include on their bottomsurface microprobes 328 for delivering electrical signals and/ormechanical stimulation to an anterior surface of the retina. Microprobes328 may include microbumps for delivering signals and/or MEMS fordelivering mechanical stimulation. Microprobes 328 may be manufacturedemploying a durable and relatively inert material such as aluminum,titanium, platinum, platinum/iridium alloy, and the like. In partbecause the delivery of an electrical signal in a saline environment mayincrease the corrosion of bare metal over time, microprobes 328 may becoated with a material, such as teflon, resin, exposy, plastic, and thelike. Additionally, the materials employed in the construction ofmicroprobes 328 may take into consideration various parameters,including a temperature, a pH level, or a salinity of vitreous humorfilling the space in the eye.

Temperature sense circuit 322 may be arranged to monitor a temperatureof prosthetic retinal devices 318A and 318B. In one embodiment,temperature sense circuit 322 may be integrated with process and controlcircuitry 324. In another embodiment, temperature sense circuit 322 maybe provided on a second substrate that is attached to a first substratethat includes process and control circuitry 324. Temperature sensecircuit 322 may further include circuitry that is arranged to activateheat transfer device 326, if a predetermined temperature is exceeded.

Heat transfer device 326 is arranged to receive heat from an uppersurface of a first substrate that is disposed away from the anteriorsurface of the retina, and to dissipate the heat toward the iris or“front” end of the eye. In one embodiment, heat transfer device 326 mayinclude a Peltier junction that is formed by the joining of twodissimilar semiconductor materials. A Peltier junction takes advantageof the Peltier effect between two dissimilar types of metals orsemiconductors that transfer heat, if they are in physical contact and acurrent is passed through them. Each type of semiconductor has its ownPeltier coefficient P. When the semiconductors are attached to eachother and a current is applied, the heat energy “Q” that is transferredby the semiconductors may be expressed as follows:Q=(P _(A) −P _(B))*I _(AB),  [1]where P_(A) is the Peltier coefficient of semiconductor A, P_(B) is thePeltier coefficient of semiconductor B, and I_(AB) is the currentflowing from semiconductor A to semiconductor B. Accordingly, byreversing the direction of the current, the direction of heattransfer/exchange may be reversed.

As described in more detail in FIGS. 4A and 4B, one embodiment of heattransfer device 326 may comprise a current supply for the heat pump thatis separate from a power supply of process and control circuitry 324. Inanother embodiment, a supply current for process and control circuitry324 may flow through the heat pump first, and the current may bereversed, if the predetermined temperature is exceeded resulting in heatto be withdrawn from process and control circuitry 324 and dissipated tovitreous humor in the direction of the iris or “front” end of the eye.

Because a density and a distribution of photosensitive cells (rods andcones) vary throughout the anterior surface of the retina, visualstimulation in a healthy eye may not be uniform across the same surface.Therefore, different types of prosthetic retinal devices 318A and 318Bwith varying microprobe densities may be implanted in different areas ofthe anterior surface of the retina.

FIG. 4A illustrates diagram 400 showing one embodiment of a temperaturecontrolled prosthetic retinal device over the anterior surface of theretina 406. Diagram 400 includes temperature sense circuit 422, heattransfer device 426, process and control circuitry 424, microprobes 428,anterior surface of the retina 406, ganglion cells 408, electricalstimulation field 436, and voltage supply 438 for process and controlcircuitry 424. Heat transfer device 426 includes current supply 432, andPeltier junction 430.

As described above, microprobes 428 may be aligned with the bottomsurface of process and control circuitry 424, engaging anterior surfaceof the retina 406, in an on-position. For optimum electricalstimulation, physical contact of each microprobe 428 with the anteriorsurface of retina 406 is preferred. Furthermore, the physical contact ofmicroprobes 428 with the anterior surface of retina 406 may provideadditional mechanical stimulation. Thus, different levels and types ofstimulation may be accomplished depending on whether microprobes 428 arein contact with anterior surface of retina 406, a pressure of thecontact by microprobes 428, and a level of stimulation current appliedby microprobes 428.

Process and control circuitry 424 is arranged to receive opticalstimulation signals, and to determine an amount of stimulation currentand a pressure to be applied by microprobes 428. In one embodiment, anambient light level and available power for process and controlcircuitry 424 may be used to determine a pressure level for contactbetween microprobes 428 and anterior surface of the retina 406.Accordingly, the pressure applied by each microprobe may be modified toachieve optimum stimulation without causing damage to the retina.

In addition, a randomly patterned duty-cycle may be applied to theelectrical and mechanical stimulation. For example, once optimumpositions of microprobes 428 and the amount of stimulation current isdetermined, the microprobes may be randomly disengaged from the retinaand reengaged. Similarly, the electrical current may be duty-cycled witha random pattern generating an optimum amount of electrical field 436for stimulating ganglion cells 408. Power for process and controlcircuitry 424 may be provided by voltage supply 438. In anotherembodiment, a current supply may provide power to process and controlcircuitry 424.

Heat transfer device 426 includes Peltier junction 430, which isarranged to be in thermal contact with process and control circuitry424. In one embodiment, current source 432, which is controlled bytemperature sense circuit 422, is arranged to provide supply current toPeltier junction 430. Temperature sense circuit 422 may be configured tomonitor a temperature of process and control circuitry 424, and activatecurrent source 432 (activating Peltier junction 430), if the temperatureof process and control circuitry 424 exceeds a first predeterminedlimit.

Peltier junction 430 is arranged to transfer heat from a surface ofprocess and control circuitry 424 to the vitreous humor by enabling theoperation of the Peltier heat transfer effect. Peltier junction 430 maycomprise a pair of metals, metal alloys, or semiconductors that transferheat in one direction based on a polarity of the supply current passingthrough them. By transferring heat from the process and controlcircuitry 424 to the vitreous humor in a direction of the iris, Peltierjunction 430 enables dissipation of the heat generated by the prostheticretinal device away from the retina and thereby at least reducing a riskof infection or immune response to an elevated eye temperature. Peltierjunction 430 is discussed in more detail below in conjunction with FIG.5.

Temperature sense circuit 422 may be further arranged to deactivatecurrent source 432 (deactivating Peltier junction 430), if thetemperature of process and control circuitry 424 drops below a secondpredetermined limit. In one embodiment, the first predetermined limitand the second predetermined limit may be selected such that ahysteretic operation of Peltier junction 430 is enabled.

FIG. 4B illustrates diagram 440 showing another embodiment of atemperature controlled prosthetic retinal device over the anteriorsurface of retina 406. Diagram 440 includes temperature sense circuit422, heat transfer device 426, process and control circuitry 424,microprobes 428, anterior surface of retina 406, ganglion cells 408,electrical stimulation field 436, and power connections 444 and 445between process and control circuitry 424 and the heat transfer device.Heat transfer device 426 includes current supply 432, inverters 442 and444, and Peltier junction 430.

Process and control circuitry 424 and microprobes 428 that are similarlynamed in FIG. 4A operate in substantially the same way as discussedabove. The supply current from current source 432 may be provided toprocess and control circuitry 424 through inverters 442 and 443, andPeltier junction 430.

Because current source 432 is arranged to power process and controlcircuitry 424, current source 432 is activated during an entireoperation of process and control circuitry 424. Accordingly, Peltierjunction 430 is also active so long as process and control circuitry 424operates. Temperature sense circuit 422 is arranged to control currentsource 432 and inverters 442 and 443, and enable a reversal of thesupply current before and after process and control circuitry 424.

In a typical operation, current source 432 may provide a predeterminedpolarity of current, and inverters 442 and 443 may be in a non-invertingmode. Peltier junction 430 may transfer heat from vitreous humor toprocess and control circuitry 424 in this mode. Vitreous humor may havea characteristic temperature of approximately 96 degrees (1-2 degreesbelow body temperature). If a temperature of the process and controlcircuitry 424 is above the temperature of vitreous humor, but below thefirst predetermined limit, the heat transferred by Peltier junction 430may help cool process and control circuitry 424.

If the temperature of process and control circuitry 424 exceeds thefirst predetermined limit, temperature sense circuit 422 may enablecurrent source 432 to reverse the supply current resulting in a reversalof heat transfer direction of Peltier junction 430. Accordingly, heatpump may begin transferring heat from process and control circuitry 424to vitreous humor in a direction of the iris as described before.

Because the supply current, which is provided to Peltier junction 430,is also provided to process and control circuitry 424 through powerconnections 444 and 445, a reversal of the supply current provided toprocess and control circuitry 424 may not be desired. A polarity of thesupply current through process and control circuitry 424 may bemaintained by activating inverters 442 and 443, which reverse the supplycurrent before and after process and control circuitry 424.

If the temperature of process and control circuitry 424 drops below thesecond predetermined limit, an operation of Peltier junction 430 may bereversed yet again by reversing the supply current at current source432. Inverters 442 and 443 may be deactivated substantiallysimultaneously with the reversal of the supply current.

In one embodiment, temperature sense circuit 422, heat transfer device426, and process and control circuitry 424 may be implemented in thesame microchip. In another embodiment, all three devices or acombination of two of the devices may be implemented in differentmicrochips.

FIG. 5 illustrates an embodiment of heat pump 530. Heat pump 530 may beimplemented as one embodiment of Peltier junction 430 in FIG. 4B. Heatpump 530 includes isolators 552, conductors 551, n-type semiconductors554, p-type semiconductors 556, and power connections 546 and 548. FIG.5 also shows direction of heat transfer 558 toward heat pump 530 anddirection of heat transfer 560 away from heat pump 530, when a supplycurrent flows from power connection 548 to power connection 546.

To implement the Peltier effect, heat pump 530 includes p-type andn-type semiconductors 556 and 554, which are mounted successivelybetween conductors 551. P-type and n-type semiconductors 556 and 554form p-n and n-p junctions that transfer heat in a direction determinedby the direction of the supply current. Each junction is in thermalcontact with conductors 551. When the supply current is applied, atemperature difference forms between conductors 551, which operate asradiators. According to the Peltier equation, the temperature differencemay be expressed as follows:

$\begin{matrix}{{T = {\frac{3}{2}{k\left( {P_{n} - P_{p}} \right)}I_{np}}},} & \lbrack 2\rbrack\end{matrix}$where P_(n) is the Peltier coefficient of n-type semiconductor 554,P_(p) is the Peltier coefficient of p-type semiconductor 556, and I_(np)is the supply current flowing from n-type semiconductor 554 to p-typesemiconductor 556. If the supply current is reversed to flow from p-typesemiconductor 556 to n-type semiconductor 554, the same temperaturedifference may be obtained, but in a reverse direction.

Equations [1] and [2] above describing Peltier effect include constantcoefficients that depend on characteristics of n-type semiconductor 554and p-type semiconductor 556. Therefore, the constant coefficients mayvary as new technology is developed and characteristics of semiconductormaterials change with new manufacturing techniques. The Peltier effect,however, may continue to be employed to exploit characteristics ofsemiconductors to exchange heat even if the equations change.

Isolators 552 are arranged to provide protection for conductors 551, andto prevent unintentional electrical contact between heat pump 530 andanother circuit. A variety of metal, metal alloy, and semiconductorpairs may be employed in heat pump 530. Fe-constantan, Cu—Ni,Pb-constantan are examples of metal-metal alloy pairs. Bi₂Te₃—Bi₂Se₃,Bi₂Te₃—Sb₂Te₃, CrAu—Bi₂Te₃, embedded into silicon, are examples ofsemiconductor pairs that may be used in a Peltier based heat pump.Implementation of heat pump 530 is, however, not limited to thesematerials. Any metal, metal alloy, and semiconductor pair, which mayprovide a desired heat transfer efficiency may be utilized in heat pump530.

An efficiency of heat pump 530 may be defined by a Coefficient ofPerformance (COP). COP may be expressed as:

$\begin{matrix}{{{COP} = \frac{Q_{1}}{W}},} & \lbrack 3\rbrack\end{matrix}$where Q₁ is an amount of heat energy withdrawn from the environment byone side of heat pump 530, and W is an energy provided to heat pump 530by the supply current. Q₁ and W are related through:Q ₂ =Q ₁ +W,  [4]where Q₂ is an amount of heat energy dissipated to the environment byanother side of heat 530. For example, at a COP of three, one Joule ofenergy provided to heat pump 530 may result in withdrawal of threeJoules of heat energy by the cooling side, and dissipation four Joulesof heat energy from the heating side.Flow Charts

FIG. 6 illustrates a flow diagram generally showing process 600 forcontrolling a temperature of a prosthetic retinal device. Moving from astart block to block 602, a temperature of the prosthetic retinal deviceis monitored. Stepping to decision block 604, a determination is made asto whether a first predetermined temperature limit is exceeded. Iffalse, the processing loops back to block 602 and performs substantiallythe same actions discussed above again.

However, if the determination at decision block 604 is affirmative, theprocess proceeds to block 606 where a heat transfer device is activatedby providing a supply current to the device. As described previously,the heat transfer device may comprise two different kinds of metals orsemiconductors that transfer heat, based on a direction of the supplycurrent flowing through the heat pump. In one embodiment, the heattransfer device may be in thermal contact with a surface of theprosthetic retinal device, and the direction of the current may beselected such that heat is transferred from the surface of the retinaldevice towards the vitreous humor and front of the eye.

Next, the process proceeds to block 608 where the temperature of theprosthetic retinal device is monitored while the heat transfer device isin operation. The process advances to decision block 610 where adetermination is made as to whether the temperature of the prostheticretinal device has dropped below a second predetermined limit. If thedetermination is negative, processing loops back to block 608 forfurther monitoring of the temperature while the heat transfer device isoperating.

However, if the decision at decision block 610 is affirmative, theprocess proceeds to block 612 where the heat transfer device isdeactivated by terminating the supply current. Next, the process returnsto block 602 and performs substantially the same actions as discussedabove. Additionally, in one embodiment, the first predetermined limitand the second predetermined limit may be selected such that a heattransfer device operates in a hysteretic mode.

FIG. 7 illustrates a flow diagram generally showing process 700 forcontrolling a temperature of a prosthetic retinal device. Moving from astart block, the process steps to block 702, where a supply current forthe prosthetic retinal device is provided to a heat transfer device. Inone embodiment, the heat transfer device and the retinal device may beformed on two substrates that are attached to each other. Providing thesupply current for the retinal device through the heat transfer devicemay enable continuous transfer of heat while the retinal device isoperating.

If a temperature of the retinal device is above a temperature of thevitreous humor, but below a first predetermined limit, the heat transferdevice can transfer approximately the temperature of the vitreous humorto the retinal device and thereby providing a cooling effect on theanterior surface of the retina. Processing then proceeds to block 704where a temperature of the retinal device is monitored and comparedagainst a first predetermined limit.

Next, processing proceeds to decision block 706 where a determination ismade as to whether the temperature of the retinal device exceeds thefirst predetermined limit. If the determination is negative, the processloops back to block 704 for further monitoring of the temperature.However, if the determination at decision block 706 is affirmative, theprocess proceeds to block 708 where a polarity of the supply current isreversed at a current source that is included in the heat transferdevice. In one embodiment, a temperature sense device such astemperature sense circuit 422 of FIG. 4B may control the current supplyand reverse the supply current upon detection of the temperatureexceeding the first predetermined limit.

Moving from block 708, the process steps to block 710 where the supplycurrent is reversed before and after the retinal device. As describedpreviously, the supply current may be provided to the retinal devicethrough a heat transfer device. Two inverter devices such as inverters442 and 443 of FIG. 4B may provide for reversal of supply current beforeand after the retinal device resulting in maintenance of a polarity ofthe supply current through the retinal device irregardless of thepolarity of the supply current through a heat pump.

Next, the process proceeds to block 712 where the temperature of theretinal device is monitored and compared against a second predeterminedlimit. Processing then proceeds to decision block 714 where a decisionis made as to whether the temperature of the retinal device drops belowthe second predetermined limit. If the determination is negative, theprocess returns to block 712 for further monitoring of the temperature.However, if the determination is affirmative, the process proceeds toblock 716.

At block 716, the polarity of the supply current is again reversed atthe current source. Processing then proceeds to block 718, where theinverters reverse the supply current before and after the retinal deviceto maintain the polarity of the current through the retinal device.Next, the process then returns to a calling process to perform furtheractions.

Each block of the flowchart illustrations discussed above, andcombinations of blocks in the flowchart illustrations above, can beimplemented by computer program instructions. These program instructionsmay be provided to a processor to produce a machine, such that theinstructions, which execute on the processor, create means forimplementing the actions specified in the flowchart block or blocks. Thecomputer program instructions may be executed by a processor to cause aseries of operational steps to be performed by the processor to producea computer-implemented process such that the instructions, which executeon the processor, provide steps for implementing the actions specifiedin the flowchart block or blocks.

Accordingly, blocks of the flowchart illustrations support combinationsof means for performing the specified actions, combinations of steps forperforming the specified actions and program instruction means forperforming the specified actions. It will also be understood that eachblock of the flowchart illustrations, and combinations of blocks in theflowchart illustrations, can be implemented by special purposehardware-based systems, which perform the specified actions or steps, orcombinations of special purpose hardware and computer instructions.

The above specification, examples and data provide a description of themanufacture and use of the composition of the invention. Since manyembodiments of the invention can be made without departing from thespirit and scope of the invention, the invention also resides in theclaims hereinafter appended.

We claim:
 1. A prosthetic retinal apparatus comprising: a circuitresponsive to light information and adapted to generate retinastimulation signals; a plurality of microprobes adapted to connect thecircuit to an anterior surface of the retina and in response to theretina stimulation signals to stimulate the retina; and a heat transferdevice coupled to the circuit and adapted to transfer heat from thecircuit and dissipate the heat from the circuit away from the retina. 2.The apparatus of claim 1, further comprising a temperature sensoradapted to sense a temperature associated with the circuit, wherein thetemperature sensor is adapted to activate the heat transfer device ifthe sensed temperature is greater than a first predeterminedtemperature.
 3. The apparatus of claim 2, wherein the temperature sensoris further adapted to deactivate the heat transfer device if the sensedtemperature is less than a second predetermined temperature such thathysteretic operation is enabled.
 4. The apparatus of claim 2, whereinthe heat transfer device is adapted to control a supply current for theheat transfer device such that heat is transferred in a first directionwhen the supply current is of a first polarity and in a reverseddirection when the supply current is of a reversed polarity; wherein thesupply current is provided at the first polarity if the sensedtemperature is below the first predetermined temperature; and wherein apolarity of the supply current is reversed if the sensed temperature isgreater than the first predetermined temperature.
 5. The apparatus ofclaim 4, wherein the supply current is provided by at least one of acurrent source circuit included in the heat transfer device and anexternal current source employing radio frequency (RF) induction.
 6. Theapparatus of claim 1, wherein the heat transfer device is adapted totransfer heat from the circuit based, at least in part, on a currentthat flows through the heat transfer device.
 7. The apparatus of claim1, further comprising a source of supply current series coupled to thecircuit and the heat transfer device such that the supply current flowsin series through the circuit and the heat transfer device, the circuithaving an input and an output; first and second inverters interposedrespectively at the input and the output of the circuit; and atemperature sensor adapted to sense a temperature associated with thecircuit; wherein the heat transfer device is adapted to transfer heat ina first direction when the supply current is of a first polarity, and ina reversed direction when the supply current is of a reversed polarity;and wherein the source of supply current and the first and secondinverters are cooperatively controlled in response to the sensedtemperature such that, when the sensed temperature exceeds a firstpredetermined temperature, (a) the supply current through the heattransfer device is reversed in polarity such that the transfer of heatby the heat transfer device is in the reversed direction, and (b) thesupply current through the circuit remains of the first polarity.
 8. Theapparatus of claim 1, wherein the heat transfer device includes aPeltier junction device thermally coupled to the circuit.
 9. Theapparatus of claim 1, wherein a coefficient of performance of the heattransfer device is at least three.
 10. The apparatus of claim 1, whereinthe plurality of microprobes includes at least oneMicro-Electromechanical System (MEMS).
 11. The apparatus of claim 1,wherein the circuit is further adapted to generate stimulation signalscomprising at least one of a stimulation current and a pressure that isapplied by the plurality of microprobes based, at least in part, on atleast one of: an ambient light level and an intra-ocular pressure. 12.The apparatus of claim 11, wherein the plurality of microprobes are eachis adapted to be disengaged and re-engaged from contact with theanterior surface of the retina based, at least in part, on a duty-cycle.13. The apparatus of claim 11, wherein the stimulation current isduty-cycled based on a predetermined rate.
 14. The apparatus of claim 1,further comprising a receiver adapted to receive power from a remotesource, the receiver adapted to employ at least one of: coherent lightand a Radio Frequency (RF) signal to receive power.
 15. The apparatus ofclaim 1, further comprising a coating disposed on at least a portion ofthe apparatus, wherein the coating includes at least one of: teflon,heparin, plastic, resin, and epoxy.
 16. A method for controlling atemperature of a prosthetic retinal device, comprising: receiving lightinformation; using the light information to generate retina stimulationsignals; stimulating the retina through a plurality of microprobescoupled to an anterior surface of a retina; wherein the retinastimulation signals are based upon the received light information; andenabling a heat transfer device to dissipate at least a portion of heatresulting from generating the retina stimulation signals away from theretina.
 17. The method of claim 16, wherein enabling the heat transferdevice to dissipate at least the portion of the heat resulting fromgenerating the retina stimulation signals comprises: providing a supplycurrent to the heat transfer device if a temperature associated with theprosthetic retinal device is greater than a first predeterminedtemperature, thereby enabling the heat transfer device to dissipate atleast a portion of the heat resulting from generating the retinastimulation signals away from the retina.
 18. A system for controlling atemperature of a prosthetic retinal device, comprising: an image sensoradapted to receive light information and convert the light informationinto an electrical signal; a circuit adapted to receive the electricalsignal and in response generate retina stimulation signals; a pluralityof microprobes adapted to connect the circuit to an anterior surface ofa retina and in response to the retina stimulation signals to stimulatethe retina; and a heat transfer device coupled to the circuit andadapted to transfer heat from at least the circuit and to dissipate suchheat away from the retina.
 19. The apparatus of claim 18 furthercomprising a temperature sensor adapted to sense a temperatureassociated with at least the circuit, and in response to control theheat transfer device to transfer heat from at least the circuit when thesensed temperature is greater than a first predetermined temperature,and to stop transferring heat from at least the circuit when the sensedtemperature is less than a second predetermined temperature, such thathysteretic operation is enabled.
 20. The system of claim 18, furthercomprising: a source of supply current coupled in series to the circuitand the heat transfer device such that the supply current flows inseries through the circuit and the heat transfer device, the circuithaving an input and an output; first and second inverters interposedrespectively at the input and the output of the circuit; and atemperature sensor adapted to sense a temperature associated with thecircuit; wherein the heat transfer device is adapted to transfer theheat in a first direction when the supply current is of a firstpolarity, and in a reversed direction when the supply current is of areversed polarity; and wherein the source of supply current and thefirst and second inverters are cooperatively controlled in response tothe sensed temperature such that, when the sensed temperature exceeds afirst predetermined temperature, (a) the supply current through the heattransfer device is reversed in polarity such that the transfer of theheat by the heat transfer device is in the reversed direction, and (b)the supply current through the circuit remains of the first polarity.